Xerographic development



April 1959 v R. E. HAYFORD 2,880,699

XEROGRAPHIC DEVELOPMENT Original Filed June 16, 1955 INVENTOR. RICHARD E. HAY FORD BY PM A ATTORNEY United States Patent XEROGRAPHIC DEVELOPMENT Richard E. Hayford, Pittsford, N.Y., assignor to Haloid Xerox lnc., Rochester, N.Y., a corporation of New York Continuation of application Serial No. 515,885, June 16, This application October 21, 1957, Serial No. ,512

6 Claims. (Cl. 118-637) This invention relates in general to xerography and in particular to new methods, means and apparatus for the development of an electrostatic latent image such as, for example, a xerographic latent image and is a continuation of my patent application Serial No. 515,885, filed June 16, 1955, and now abandoned.

In the art of xerography, as originally disclosed in Carlson US. Patent 2,297,691, an electrostatic latent image, otherwise called a xerographic latent image, is formed on an insulating surface, such as generally a photoconductive insulating layer, by the combined action of electric field and a pattern of activating radiation, such as light. It is usual in the art of xerography to transform this latent image to a visible image, such as reproduction of a scene or two-dimensional original, by the deposition on the electrostatic image bearing surface of finely divided electroscopic particles. As disclosed by Carlson, dust or like material may be deposited on the image bearing surface by electrical attraction of oppositely charged powder material, or in tone reversal, by deposition of material of the same polarity as the image.

Subsequently there was shown in Wise Patent 2,618,552 the development of a xerographic latent image by cascading across the image surface a two-component developer consisting of carrier particles bearing on the surface oppositely charged powder particles. This invention in xerographic development improved the quality and speed of xerographic development to such an extent as to make commercially practical many uses and applications of xerography which were theretofore either impossible or not feasible. In particular the cascade method of xerographic development is particularly valuable for the development of line copy images, inasmuch as it produces sharp black lines on an extremely white background substantially completely free from background haze caused by deposition of powder in the background areas. This system, however, had several drawbacks, one of -which was the mechanical difficulty in continuously or repeatedly cascading the developer downward across a plate, inasmuch as it then becomes necessary for a subsequent stage in the operation to return the developer upwardly to its starting position. In addition, in cascade development the proportions of ingredients, namely the carrier and the developer powder, must be maintained by continual or continuous addition of small quantities of powder to replace that which is consumed in the xerographic development.

It has also been found for many purposes, and particularly for comtinuous tone images wherein it is desired to reproduce varying shades of gray in a xerographic print, that the xerographic image may advantageously be developed by deposition of powder in conjunction with a very closely spaced counterelectrode during development so as to reduce or eliminate distortion in the electric field associated with the xerographic latent image. As disclosed in co-pending application Serial No. 185,387, it is desirable for this counterelectrode to be maintained at extremely close spacing, such as, for example, closer 2,880,699 Patented Apr. 7, 1959 'ice than ,4 inch, during the development operation and it is desirable where possible to maintain the spacing as close as possible within this threshold maximum of ,4 inch. Such spacing requirements obviously create problems in mechanical operation, since it is necessary to maintain extremely close dimensions and to maintain surfaces close together and clean. Obviously, maintenance of cleanliness of closely adjacent facing surfaces is a difiicult problem.

Now in accordance with the present invention, a xerographic latent image is developed by means, methods and apparatus including an electrostatic development member having developing powder loosely adherent by electrostatic forces to the surface of a thin insulating film backed by a conductive potential applying member.

Further in accordance with the present invention, a xerographic latent image is developed by passing across and in contact with the image bearing surface an electrostatic developer member comprising a thin insulating film electrically carrying charged developer powder and backed by a flexible counterelectrode adapted to be passed along the surface of the image layer at a distance spaced from such surface only by the thickness of the insulating layer and by the developed image itself.

It is, therefore, an object of the invention to provide new apparatus, method and means for the development of a xerographic latent image.

It is an additional object of the invention to provide new apparatus, methods and means for the xerographic development of an electrostatic image on an insulating or photoconductive insulating surface by means of loosely and adherently held finely divided developer powder particles secured to and deposited onto the image surface by an insulating film backed by a biased conductive member.

It is another object of the invention to provide new and improved xerographic apparatus including developing means, methods and apparatus according to certain embodiments of the present invention.

Additional objects will in part be obvious and will in part become apparent from the following specification and drawings, in which:

Figure 1 is a perspective view partly in section of a developing member according to one embodiment of the invention;

Figure 2 is a diagrammatic view illustrating one step of the present invention;

Figure 3 is a similar diagrammatic view illustrating another step of the present invention;

Figure 4 is a perspective view of another embodiment of a developing member according to the present invention;

Figure 5 is a diagrammatic view of apparatus for developer loading according to another embodiment of the invention; and

Figure 6 is a diagrammatic view of a xerographic machine according to one embodiment of the present invention.

In Figure 1 there is illustrated, diagrammatically, a development member according to one embodiment of the invention. As illustrated in this figure the development member generally designated 10 may comprise a support handle 11 or the like particularly adapted for manual operation. Desirably the support handle or at least a working portion of the handle should be of an insulating material in order to protect the operator from electrical hazard and may, for example, be wood, plastic, or the like. Obviously, if other means of protection are employed, it may be constructed of an electrically conductive material such as metal. Secured to the lower end of the support handle 11 is a development loop 12 formed in the shape of a loop,.ball, bag, or the like.

This loop comprises an outer insulating film 14 and an inner conducting film 15. Optionally, the insulating and conductive layers are preformed by applying a conductive coating to a sheet of an insulating film. One particular film which has been employed with desirable results consists of an extremely thin film about /2 mil in thickness of polyethylene terephthalate, available under the name Mylar. One surface of this film is coated with a thin, metallic layer such as, for example, a thin evaporated aluminum layer. This particular combination of Mylar film conductively coated with a thin metallic layer has a peculiar value for the present invention because of the extreme thinness coupled with strength. Being thin, the film is extremely flexible and pliable and has a very soft texture and pressure. In addition, it permits the conductive backing film 15 to be brought into virtual contact with an adjacent surface during operation.

Further illustrated in Figure 2 is the operation of the member of Figure 1 for loading with development particles. In this figure the development member 10 is positioned above a loading plate 16 which bears developer particles 17. The conductive layer 15 of the development member is electrically connected through a DC. power source 19 to the loading plate 16, for example, by means of an electric load connected to the inside or conductively coated side of the development loop. As illustrated in the figure the development member 1'3 is being brushed across the surface of the loading plate id to pick up powder particles secured loosely and electrostatically to the outer surface of the development loop In Figure 3 is illustrated the operation of the development member to deposit developer particles on a xerographic latent image. In this figure the development member It) is positioned above with its development loop in brushing contact with a surface of an image plate 2d which may, for example, comprise a conductive backing support 21 having an insulating layer 22 on its surface. The insulating layer may, if desired, be a photoconductive insulator and thus the image plate may be a photo'- sensitive xerographic plate. The image plate 29 is illustrated as having an electrostatic image as indicated by plus marks or positive signs 24 in image areas of its surface. The conductive layer 15 of the development loop in this case is connected through a bias power supply 25 adapted to apply a low bias potential to the conductive layer with respect to the backing plate 21.

In operation according to Figures 1, 2, and 3, the development, member 143 is first loaded with developer particles 17 on the development loop 12 by brushing it along the surface of the powder bearing loading plate 16 while maintaining a relatively high electric potential between the development member and the loading plate. This potential may be in the order of 100 volts or less, or may be up to 500 or 1000 volts and is adapted to pick up and carry charged particles from the loading plate and to retain these particles in a loosely adherent, electrically releasable condition on the surface of the development electrode. After loading in this manner the development member is employed in a brushing action to deposit particles on the xerographic plate. For powder deposition, as illustrated in Figure 3, the development member is connected through a low potential or bias source such as, for example, a ground potential or nearly ground potential adapted or adjusted to apply suflicient electric bias between the development loop and the xerographic plate so as to deposit developer particles in an image body 26 on the surface of the xerographic plate to correspond to. the image areas of the xerographic latent image. The bias potential selected for this purpose dependsupon the strength of the charge in the image and background areas of the xerographic plate. Thus, if the image and background charges are relatively low, and direct or black-for-black development is desired, the bias: potentialwill bearound volt or up to or volts; whereas if the image and background charge potcntials are relatively higher the bias potential may be up to 25, 50, or even volts. As a general matter, the bias potential should be substantially equal to or a few volts higher than, and of the same polarity as, any residual or background potential in the nonimage areas of the electrostatic latent image. Thus, for example, if the image consists of charged areas having a potential of 100 or several hundred volts in a background having residual charge potential of around 20 volts, it would generally be advisable to adjust or select the bias potential source so as to supply a bias potential of about 20 to 25 volts.

The brushing of the development member across the xerographic plate surface may be completed in a single pass of the member across the surface or may be completed with several passes until the image is judged by examination or experience to be properly developed. In this connection it is observed that the image may consist of a pattern of electric charge on a photoconductive insulating surface in which case the image is developed to a stage of completion determined by experience or by examination under nonactivating radiation such as red light or the like. The image surface, on the other hand, may consist of an insulating layer 22 which is not conductive or at least not conductive in the presence of visible light and may or may not be supported on a conductive backing member 21. In such a case develop ment may be carried out in room light with completion of development determined by visual examination.

In Figure 4 is illustrated a modified development member generally designated 49 consisting of a support bar 41 having a development loop 42 along one edge. The support bar may be adapted for manual operation but desirably is adaptable to mechanized operation. Thus, for example, the bar 41 may be adapted to be mounted on a mechanized support carrier (see Figure 6) and desirably will be constructed of conductive material such as to permit easy application of biasing or activating materials through the machine mounting. The development loop 42 desirably may be constructed of the same material as development loop 12, including a thin insulating outer film and a conductive inner film. The development loop may be mounted on a bar .1 by any suitable means such as, for example, a mounting strip dd secured by means of screws 45 or the like in pressure contact against the outer surface of the end or edges of the dc velopment loop.

In Figure 5 is illustrated an alternative developer loading apparatus and method. According to this figure the development member 10, as shown in either Figures 1 or 4, is suitably biased and held at a developer loading potential by means of loading power supply 19, and is positioned with its development loop in the path of discharge of a powder spray device generally designated 50. This device may comprise, for example, a powder particle container 51 operably connected to a gas supply tube 52 and fitted with an atomizer gas lead 54- terminating in a nozzle Contained Within the powder particle container is a supply of developer particles 17 adapted to be sprayed in a stream from the nozzle 55. Desirably the nozzle will be highly restricted \lhfil'QY powder sprayed therefrom will pass through the nozzle orifice in turbulent rather than laminar flow. Thus, the powder spray emerges from the powder spray device St) in the form of an air or gas suspension of charged powder particles at least some of which and preferably most or all are charged to the desired polarity. As illustrated in Figure 5 this spray is sprayed upon the surface of the development loop 12 which is maintained at a desired powder loading potential whereby the loop selectively attracts, for loading, powder particles 17 charged to a polarity opposite to that imposed on the development loop by power supply source 19. Thus, for xample, when the power supply is adapted to apply a.

positive polarity potential to the development loop, negatively charged developer powder particles are selectively coated onto the surface of the development loop and the positively charged particles are substantially repelled thereby.

In this connection, it is observed that the development apparatus disclosed in Figures 1 to 5 is also adapted to the process known as reversal development, or the development of black areas to correspond with white areas in the original. This, or" course, is particularly desirable when the invention is used in conjunction with photofinishing procedures or recording from certain special light sources, such as cathode ray tubes, galvanometer traces, and the like. When employed in this manner, the development member is loaded with powder particles of the same polarity as that on the image bearing surface. Thus, for example, if image plate 20 bears a positive polarity image and it is desired to reproduce this image in reversal development, power supply 19 will be maintained at negative polarity during the loading operation. Thus, in Figure 5 it is seen that application of negative polarity to the development loop during loading will cause the deposition of positively charged developer powder particles on the development loop. Then during image development, as illustrated in Figure 3, the conductive film of the development loop will be maintained at a positive polarity with respect to backing plate 21 and this positive polarity preferably will be substantially equal to the potential existing in the most highly charged image area. Under these conditions, the positively charged particles will deposit only in those image areas which are substantially less highly charged than the highest potential, and will thereby cause reversal development.

In Figure 6 is illustrated a xerographic machine operating according to the principles of the present invention. In this machine, a xerographic latent image is formed and developed on a xerographic cylinder 61, which may comprise, for example, a cylindrical or drum-like memher having a photoconductive insulating layer on its outer surface. Such photoconductive insulating layers are conventional in the art and may include materials such as vitreous selenium, anthracene, sulfur, and the like, as well as photoconductive crystals related to phosphor materials bound onto the surface in an insulating film and including, for example, the photoconductive sulfides, selenides, and oxides of cadmium, zinc, calcium, and the like. Positioned around the xerographic cylinder 61 are a plurality of xerographic stations, such as, for example, charging station 62 which may comprise a corona discharge electrode having one or more fine wires 63 within a shield 64. In such a case, the fine wires will be connected to a high voltage source 65 to apply a corona generating potential to the wires with respect to the shield. Thus, an insulating surface such as the xerographic cylinder surface passing beneath the corona discharge wires will be charged to a desired polarity and potential.

Positioned next in the direction of rotation of the cylinder is an exposure station generally designated 67, including a lens 68 adapted to project and focus an image of an exposure slit 69 through an image slit 70 onto the moving surface of the xerographic cylinder in slit exposure arrangement.

Positioned next around the xerographic cylinder is a developing station according to one or another of the embodiments of the present invention. As illustrated in the figure, the development station generally desig- Dated 72 may comprise a developing wheel or cylinder 73 bearing a plurality of development members 40 of the general type disclosed in Figure 4. These members are conductively mounted on and connected to the wheel 73 so as to be maintained at the same electric potential as the wheel. Beside the developing wheel 73 is a developer loading drum 75 which is fed by developer hopper 76 containing suitable developer particles 17. The developer hopper is adapted to distribute on the surface of the loading drum a steady supply of developer particles, which tumble across the surface and are picked up in a catching hopper 77. In passing across the surface, developer particles 17 are deposited on the drum to coat it lightly with this powder. If desired, at this point there may be employed a spray device such as the spray device 50 of Figure 5, or other loading means, such as for example, cascading or tumbling across the surface a twocomponent developer as described in the Wise patent, hereinbefore referred to. Optionally, the developer loading drum may then be carried past a charging electrode 78, which may comprise a fine corona discharge wire 63, a shield 64, and an operably connected power supply 65, as with the charging station 62 for the xerographic cylinder, though generally of opposite polarities. A charging station of this sort may, if desired, be employed in conjunction with the developer loading drum wherever or whenever it is determined that the powder particles deposited on the surface are insufiiciently or nonuniformly charged.

Positioned next in the direction of rotation of xerographic cylinder 61 is a transfer station 80, at which may be an additional corona charging electrode including one or more fine wires 63, shield 64, and an operably connected power supply 65, as in the charging station 62. A supply of a transfer web material 81, such as for example, a continuous web or roll, may be positioned from a supply source such as a feed roll 82 around guide rollers 83 in conjunction with a drive roller 84 pressing against the surface of the xerographic cylinder 61 and in contact therewith between the cylinder and the transfer station 80. The web, upon leaving the cylinder, passes through a heating oven 85 containing heater elements 86 and thence to a takeup roll 87.

Electrically the xerographic cylinder preferably is grounded for maximum safety to the operator, and the developer wheel 73 desirably is maintained at a slight bias potential with respect to the xerographic cylinder. The developer loading wheel 75 is maintained at an elevated potential, such as for example, a potential in the order of 500 to 1000 volts, in accordance with the illustration of Figure 2. The corona discharge wires 63 are maintained at the desired charging potentials, which may be, for example, in the order of several thousand volts, such as preferably 6,000 to 10,000 volts either positive, negative, or alternating potential as desired for the charging of the surface. The shield 64 desirably may be maintained at ground potential or optionally at an adjustable or predetermined potential preferably of the same polarity as the polarity of charging desired. A suitable power supply may, for example, include a transformer 90, a rectifying tube 91, filter condensers 92, a choke 93 connected in conventional circuitry to apply a potential across a potentiometer 94 provided with suitable fixed or adjustable taps as may be conventional in the electrical art.

In use and operation the cylinders are rotated by suitable drive means, preferably in the direction indicated by the arrows, to rotate each of these members through its operating cycle. A xerographic cylinder 61 is charged at the charging station to apply a sensitizing charge to the photoconductive insulating layer and is exposed at the exposure station to cause selective charge dissipation, resulting in a xerographic latent image. At the development station powder is loaded onto the loading cylinder 75, and picked up by the development bars 40 on developer wheel 73 as illustrated in Figures 1 to 5, The developer bars are then rotated in brushing contact with the xerographic cylinder 61 to deposit image forming developer powder particles thereon. At the transfer sta tion these developer particles are transferred to the web 81 to form a xerographic print, which may be fixed by heating in oven 85 or by other fixing means, such as for example, by means of solvent, pressure, or coating with a binder material or the like. A permanent xerographic plate corresponding to the copy presented by slit 69 is thus wound up on takeup roll 87, forming the output of the machine.

It is apparent that the present invention is adapted for the reproduction of xerographic prints or for the printing of electrostatic latent images of all types and is particularly useful for the production of direct or reversal copy. One of the particular advantages of the present invention is its easy convertibility from direct to reversal copy, since this conversion can be made directly by reversing polarity of power supply 19 and adjusting the bias potential of power supply 25. In addition, the invention has the advantage of bringing extremely close to the image surface the conductive layer of the development loop, whereby an enhanced development electrode action is secured. Furthermore, the softness of the electrode contributes to an excellent texture in the appearance of the developed xerographic image. Mechanically it is observed that numerous advantages are achieved in machine design and operation, not the least of which is the ability to avoid constant circulation of a two-component developer while at the same time avoiding the critical cleaning and operating conditions of ordinary powder cloud development. The quality of the image is excellent and the operation of the machine is simple.

It is to be understood that numerous variations and modifications may be made within the scope of the present invention. Thus, for example, the machine illus trated in Figure 1 may be modified for use with a single use, consumable image forming member such as a web of photoconductive coated paper or the like, and the paper may be carried through a plurality of xerographic stations, omitting transfer, as is the xerographic cylinder f Figure 6. Similarly, wide varieties of insulating films and conductive layers may be employed for the development loop, includirv virtually any insulating self-supporting film such as paper, cellophane, plastic and resin sheets, and the like. In view of these and other obvious variations, it is to be understood that the description of the invention is illustrative, not limiting, in its scope.

What is claimed is:

1. Apparatus for developing an electrostatic latent image on an insulating image bearing surface, said apparatus comprising a support member, a thin continuous insulating external substantially uniform film covering a conductive internal backing film, said films being flexibly supported on said support member, support means to support an electrostatic image bearing surface, a supply of charged, finely divided developer powder particles, a source of electric potential and means to apply said potential to tl e internal conductive backing, means to present the external insulating film into powder loading contact with said charged particles while said potential is applied to the conductive backing, said potential being sufiicient to attractively deposit said powder particles in a substantially uniform layer on the insulating external film when said film is in loading contact, means to apply to the conductive backing a relatively low bias potential with respect to the support means and the electrostatic image bearing surface, said bias potential being between substantially zero and substantially the highest electrical potential of said electrostatic image, and means to present the insulating external film into brushing contact with the image bearing surface while said bias potential is applied, whereby the charged developer particles are selectively deposited on said image bearing surface in configuration with the electrostatic image.

2. Apparatus for developing an electrostatic latent image on an insulating image bearing surface, said apparatus comprising a support member, a thin continuous insulating external substantially uniform film covering a conductive internal backing film, said films being flexibly supported on said support member, support means tosupport an electrostatic image bearing surface, a supply of charged, finely divided developer powder particles, 21 source of electric potential and means to apply said potential to the internal conductive backing, means to present the external insulating film into powder loading contact with said charged particles while a loading potential is applied to the conductive backing, said loading potential being sufficient to attractively deposit said powder particles in a substantially uniform layer on the insulating external film when said film is in loading contact, means to remove said film out of loading contact, means to apply to the conductive backing a relatively low bias potential with respect to the support means and the electrostatic image bearing surface, and means to present the insulating external film into brushing contact with the image bearing surface while said bias potential is applied, said bias potential being between substantially zero and substantially the highest electrical potential on said electrostatic image and being of a nature to release powder particles loaded to said insulating external film to the electrostatic image during brushing contact therewith thereby depositing charged developer particles selectively on said image bearing surface in configuration with the electrostatic image.

3. Apparatus for developing a xerographic latent image comprising a xerographic latent image surface, a loading station remote from said xerographic image surface and having a supply of charged finely divided xerographic'de veloper powder particles, a developer bar movable into loading contact with charged particles at the loading station and into developer depositing relationship with the xerographic image surface, said developer bar comprising a development loop of flexible, substantially uniform and thin, continuous, insulating, external film, a flexible conductive internal backing for said insulating film and support mounting means for said loop, a source of powder loading electric potential adapted to apply a loading potential to the development loop with respect to the charged particles, said loading potential being suflicient to electrostatically attract said particles and to secure said particles in a substantially uniform layer to the outer surface of the development loop, a bias potential source adapted to apply to the development loop with respect to the xerographic latent image surface a bias potential between about zero volt and about the highest electric potential of the xerographic latent image, and means to brush the powder bearing development loop into developing and brushing contact with the xerographic latent image surface while said bias potential is applied, whereby charged developer particles are deposited in image configuration conforming with the xerographic latent image.

4. Xerographic apparatus comprising in combination a xerographic cylinder having at least a portion of its surface comprising a photoconductive insulating layer overlying a conductive backing, means to rotate the cylinder past a plurality of xerographic operating stations for the formation of a xerographic latent image thereon, a xerographic development station positioned subsequent in the direction of rotation of the cylinder to said xerographic operating stations, said xerographic development station comprising a loading station remote from the xerographic cylinder and a source of charged finely divided xero graphic developer powder particles at said loading station,

at least one development bar movable between said load-' ing station and said xerographic cylinder and comprising a substantlally uniform and thin, continuous, flexible, insulatmg, external film covering a flexible, conductive, in-

ternal backing, a loading electric potential source elec-' trically connected to said conductive backing to apply to the backing a loading electric potential suflicient to attract and secure to the development bar a substantially uniform layer of oppositely charged developer powder particles, a bias potential source adapted to apply between the developed bar and the xerographic cylinder a bias potential between substantially zero and substantially the highest electric potential of the xerographic latent image and means to move the development bar from the loading station to the xerographic cylinder, and means to brush the development bar against the latent image bearing surface of the xerographic cylinder, whereby charged developer particles are loaded onto the development bar at the loading station and deposited on the xerographic cylinder in image configuration conforming with the xerographic latent image.

5. A xerographic apparatus of claim 4 wherein the bias potential source is a source of substantially zero potential between the development bar and the xerographic cylinder.

6. The xerographic apparatus of claim 4 wherein the bias potential source is a source of electric potential of substantially the voltage of the highest potential in the xerographic latent image being developed.

References Cited in the file of this patent UNITED STATES PATENTS 2,385,873 Melton Oct. 2, 1945 2,558,900 Hooper July 3, 1951 2,624,652 Carlson Jan. 6, 1953 2,811,465 Greig Oct. 29, 1957 FOREIGN PATENTS 658,699 Great Britain Oct. 10, 1951 

